The long-term objective of this work is to elucidate the molecular signaling mechanisms of the chemoreceptors that mediate chemotactic behavior in E. coli. These transmembrane receptors form stable ternary complexes with two cytoplasmic proteins - CheA, a histidine autokinase, and CheW, which couples CheA to chemoreceptor control. Most of the prodigious signal amplification that occurs in the chemotaxis pathway takes place in these receptor complexes, which are organized into highly cooperative arrays. The proposed studies address two key mechanistic questions: How do receptor molecules control and amplify CheA output signals? How do receptor molecules couple input stimulus information to output signal control? The principal subject of these studies is the serine chemoreceptor, Tsr. To investigate the mechanism of CheA control in Tsr signaling complexes, we will conduct additional tests of the trimer-of-dimers model of receptor signaling teams;characterize the trimer-forming, cluster-forming and signal amplification properties of receptors with trimer contact lesions;identify the CheW-binding and CheA control determinants in receptors by a mutational survey of all polar and hydrophobic residues in the core Tsr signaling domain;construct and characterize soluble single-chain Tsr signaling fragments for ternary complex studies;use Tsr~Tsr constructs to manipulate the geometry and stoichiometry of receptor signaling complexes;and construct mini-CheA and triplet CheW molecules to simplify and stabilize Tsr signaling complexes for high-resolution structural studies. A HAMP domain translates transmembrane piston motions into conformational changes that regulate output from the Tsr signaling domain. To elucidate the role of the HAMP domain in Tsr signaling, we will exploit a large collection of Tsr-HAMP mutants to investigate HAMP interactions with the cytoplasmic membrane;develop in vivo cysteine-directed crosslinking probes for HAMP structures and signaling states;compare the structural and functional consequences of HAMP alterations;investigate the mechanisms of HAMP input and output control with mutations that alter the length and amino acid composition of the segments that link HAMP helices to adjoining Tsr structural elements. These studies should not only provide important conceptual insights into the signaling mechanisms of bacterial chemoreceptors, but may also lead to new therapeutic approaches for combating bacterial pathogens that employ motility and chemotactic behavior during the infection process.